1. Field of the Invention
The present invention relates to medical devices for monitoring vital signs, e.g. blood pressure.
2. Description of the Related Art
Pulse transit time (‘PTT’), defined as the transit time for a pressure pulse launched by a heartbeat in a patient's arterial system, has been shown in a number of studies to correlate to both systolic and diastolic blood pressure. In these studies PTT is typically measured with a conventional vital signs monitor that includes separate modules to determine both an electrocardiogram (ECG) and pulse oximetry. During a PTT measurement, multiple electrodes typically attach to a patient's chest to determine a time-dependent ECG characterized by a sharp spike called the ‘QRS complex’. This feature indicates an initial depolarization of ventricles within the heart and, informally, marks the beginning of the heartbeat. Pulse oximetry is typically measured with a clothespin-shaped device that clips to the patient's index finger, and includes optical systems operating in both the red and infrared spectral regions. In addition to measuring a pulse oximetry value, this method yields a time-dependent waveform, called a plethysmograph, that indicates both heart rate and a time-dependent volumetric change in an underlying artery (e.g. in the finger) caused by the propagating pressure pulse.
In many studies PTT is calculated from the time separating the onset of the QRS complex to the foot of the plethysmograph. Alternatively, PTT can be calculated as the time separating signals measured by two sensors (e.g. optical or pressure sensors), each sensitive to the propagating pressure pulse, placed at different locations on the patient's body. In both cases, PTT depends primarily on arterial tone, arterial compliance, the propagation distance (closely approximated by the patient's arm length), and of course blood pressure. Typically a high blood pressure results in a shorter PTT.
A number of issued U.S. patents describe the relationship between PTT and blood pressure. For example, among others, U.S. Pat. Nos. 5,316,008; 5,857,975; 5,865,755; and 5,649,543 each teach an apparatus that includes conventional sensors that measure an ECG and plethysmograph that are processed to measure PTT. U.S. Pat. Nos. 6,511,436; 6,599,251; and 6,723,054 each teach an apparatus that includes a pair of optical or pressure sensors, each sensitive to a propagating pressure pulse, that measure PTT. As described in these patents, a microprocessor associated with the apparatus processes the PTT value to estimate blood pressure.
PTT-based measurements of blood pressure are complicated by a number of factors, one of which is the many time-dependent processes associated with each heartbeat that may correlate in a different way with blood pressure, or in fact may not correlate at all. For example, prior to the initial depolarization of the ventricles (marked by the QRS complex), the mitral valve opens and lets blood flow from the left atrium into the left ventricle. This causes the ventricle to fill with blood and increase in pressure. After the onset of the QRS, the mitral valve closes and the aortic valve opens. When the heart contracts, blood ejects into the aorta until the aortic valve closes. The time separating the onset of the QRS and the opening of the aortic valve is typically called the pre-injection period, or ‘PEP’. The time separating opening and closing of the aortic valve is called the left ventricular ejection period, or ‘LVET’. LVET and PEP, along with additional time-dependent properties associated with each heartbeat, are typically included in a grouping of properties called systolic time intervals, or ‘STIs’.
PTT and LVET can be measured with a number of different techniques, such as impedance cardiography (‘ICG’) and by measuring a time-dependent acoustic waveform, called a phonocardiogram (‘PCG’), with an acoustic sensor. The PCG, characterized by acoustic signatures indicating the closing (and not opening) of the mitral and aortic valves, is typically coupled with an ECG to estimate PEP and LVET. For example, U.S. Pat. Nos. 4,094,308 and 4,289,141 each teach an apparatus that measures a PCG and ECG, and from these waveforms estimates PEP and LVET. U.S. Pat. No. 7,029,447 teaches an apparatus using transit times calculated from an ICG measurement to determine blood pressure.
Studies have also shown that a property called vascular transit time (‘VTT’), measured from features in both a PCG and plethysmograph, can correlate to blood pressure. Such a study, for example, is described in an article entitled ‘Evaluation of blood pressure changes using vascular transit time’, Physiol. Meas. 27, 685-694 (2006). In addition, studies have shown that PEP and LVET, taken alone, can correlate to blood pressure. These studies typically require multiple sensors placed on the patient's body to measure time-dependent waveforms that are processed to determine PEP and LVET. Studies that relate these properties to blood pressure, for example, are described in ‘Systolic Time Intervals in Man’, Circulation 37, 149-159 (1968); ‘Relationship Between Systolic Time Intervals and Arterial Blood Pressure’, Clin. Cardiol. 9, 545-549 (1986); ‘Short-term variability of pulse pressure and systolic and diastolic time in heart transplant recipients’, Am. J. Physiol. Heart Circ. Physiol. 279, H122-H129 (2000); and ‘Pulse transit time measured from the ECG: an unreliable marker of beat-to-beat blood pressure’, J. App.l Physiol. 100, 136-141 (2006).